1. Field of the Invention
The present invention generally relates to the field of semiconductor devices, and more particularly, to an improved performance, lateral, high-voltage PNP transistor.
2. Related Art
The design of lateral PNP transistors is complicated by the need to produce transistors which satisfy various performance requirements. One requirement is for high-voltage operation. Such devices must also produce sufficient current gain. Because the lateral PNP transistor (also referred to herein as "the transistor") is designed for high-voltage capability, the base drift region of the device must be lightly doped to sustain these high voltages. This low doping in the base drift region causes the base drift region to be vulnerable to electrostatic build-up in the overlying oxide. Such build-up can result in excessive off-state leakage current, degradation in breakdown voltage and a reduction in current gain from the overlying oxide into the base drift region. Thus, a further requirement is the need for full electrostatic charge shielding of the base drift region from chargers that can exist primarily in the dielectric layers (e.g., silicon dioxide layers) on top of the silicon surface.
To satisfy these requirements, a polycrystalline silicon shield (hereafter called a polysilicon shield) is used to protect the base drift region from presence of charges from the overlying the dielectric layers. However, lateral, high-voltage PNP transistors formed according to a BCDMOS (bipolar complementary double diffusion metal oxide silicon) technology demonstrate a kink in the I.sub.C -V.sub.CE characteristic curves and low early voltage due to the potential of the polysilicon field plate needed to electrically shield the base surface region. (See "An Analog/Digital BCDMOS Technology with Dielectric Isolation--Devices and Processes," Lu et al., I.E.E.E. Transactions on Electron Devices, Vol. 35, No. 2, p. 230-239 (Feb. 1988).) The kink in the collector-emitter characteristics of the transistor disturbs the output impedance and current gain in the active region of operation of the transistor, and lower early voltage.
Thus, what is desired is a lateral, high-voltage PNP transistor having a structure which eliminates the "kink" demonstrated by existing devices.